(1) Field of the Invention
The present invention relates to a polymer LP (Line Post) insulator comprising an FRP (Fiber Reinforced Plastics) rod, an insulation overcoat member made of rubber, which is arranged on an outer surface of the FRP rod, and securing metal members clamped at both end portions of the FRP rod, wherein an end portion of the insulation overcoat member is inserted into an end portion of the securing metal member, and also relates to a method of producing a polymer LP insulator having a construction mentioned above.
(2) Related Art Statement
Generally, a polymer LP insulator has been known as a kind of insulators. FIG. 5 is a schematic view showing one construction of the known polymer LP insulator. In the embodiment shown in FIG. 5, a polymer LP insulator 51 comprises an FRP rod 52, an insulation overcoat member 53 made of rubber, which is arranged on an outer surface of the FRP rod 52, and securing metal members 54 clamped at both end portions of the FRP rod 52. In addition, an end portion of the insulation overcoat member 53 is inserted into a space between an end portion of the securing metal member 54 and the FRP rod 52.
The construction of the polymer LP insulator 51 mentioned above is not so different from that of insulators. However, as shown in FIG. 6, the polymer LP insulator 51 is secured to a pole 64 in an inclined manner by means of a securing holder 62, when it is actually used for supporting a transmission line 61. Therefore, since a bending force is applied to a line supporting end of the polymer LP insulator 51, a maximum compression/tensile stress due to such a bending force is generated at a fulcrum point of the FRP rod 52 of the polymer LP insulator 51. In addition, a shearing stress is concentrated at the FRP rod 52 corresponding to a supporting portion of the securing metal member 54 arranged at a pole side.
As shown in FIG. 5 in detail, in the known polymer LP insulator 51 having the construction mentioned above, an end portion of the insulation overcoat member 53 is inserted into an end portion of the securing metal member 54. In this case, there is a space 63 defined by the FRP rod 52, an end portion 53a of the insulation overcoat member 53, and an inner surface of an end portion 54a of the securing metal member 54. Therefore, if a bending force is applied to a top portion of the polymer LP insulator 51, a stress is liable to be concentrated at a starting point of the space 63 i.e. a connection point 54b between the securing metal member 54 and the FRP rod 52, and thus there arises a problem such that micro fractures are sometimes generated.
Moreover, the polymer LP insulator 51 having the construction shown in FIG. 5 is produced by the steps of forming the insulation overcoat member 53 made of rubber on an outer surface of the FRP rod 52, arranging the securing metal member 54 at an end of the FRP rod 52, and clamping and fixing the securing metal member 54 to the FRP rod 52 by applying a pressure to the securing metal members 54. In the clamping and fixing step mentioned above, if there is a space 63 defined by the FRP rod 52, an end portion 53a of the insulation overcoat member 53, and an inner surface of an end portion 54a of the securing metal member 54a mentioned above, there also arises a problem such that a stress is liable to be concentrated at a starting point of the space 63 i.e. a connection point 54b between the securing metal member 54 and the FRP rod 52.
According to various factors in an actual use or in a producing step as mentioned above, if a pressure if applied to the top portion of the polymer insulator 51, micro fractures are sometimes generated near the connection point 54b between the securing metal member 54 and the FRP rod 52 even under about 50% of an assuring pressure to which the polymer insulator 51 must be endured. Therefore, the known polymer insulator 51 has a room for improving a long term assuring pressure.
In addition, in order to reduce a stress of the securing portion of the polymer insulator, the present applicant discloses a technique in Japanese Laid-Open Publication No.5-274936 such that a cylindrical stress concentration relief member made of FRP is arranged at an open end portion of an inner surface of the securing metal member. However, the technique for the known polymer insulator disclosed in Japanese Patent Laid-Open Publication No.5-274936 is not sufficient to solve the problems mentioned above for the polymer LP insulator to which a bending pressure is applied, and thus the problems mentioned above can not be solved.